Is it that one has to start with the tar ball from John's site, to start with the
pure64 installation?

Yes.

Isn't there some way of picking up the required packages from
alioth's pure64 repository?

You are _supposed_ to be able to start it from alioth's repository, but
I haven't tried it because
(1) I don't have time to reinstall my existing pure64 from scratch (and
lose its usefulness for the duration)
(2) I don't have spare disk space on the laptop to bring up a fourth
independent debian installation.

Perhaps the biggest problem with pure64 is ... it now works well enough
to be used normally.
I have to schedule time for development (the kind that needs reboots)
among primary activities.